Create fourslash syntax, compiler testcase, and build instructions summary

.github/copilot-instructions.md

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+# TypeScript Test Writing Guide for Copilot
+
+This document provides a concise guide for writing TypeScript fourslash tests and compiler tests, along with build instructions.
+
+## Build Instructions Summary
+
+### Setup
+1. Install Node.js (current or LTS)
+2. Install hereby: `npm install -g hereby`
+3. Clone the repository: `git clone --depth=1 https://github.com/microsoft/TypeScript`
+4. Install dependencies: `npm ci`
+
+### Common Build Tasks
+```bash
+hereby local             # Build the compiler into built/local
+hereby clean             # Delete the built compiler  
+hereby tests             # Build the test infrastructure
+hereby runtests          # Run all tests
+hereby runtests-parallel # Run tests in parallel (recommended)
+hereby runtests --runner=fourslash # Run only fourslash tests
+hereby runtests --runner=compiler # Run only compiler tests
+hereby runtests --tests=<testPath> # Run specific test
+hereby baseline-accept   # Accept new test baselines
+hereby lint              # Run eslint
+hereby format            # Run code formatting
+```
+
+## Fourslash Test Syntax Guide
+
+Fourslash tests are interactive TypeScript language service tests. They validate IDE features like completions, quick info, navigation, and refactoring.
+
+### Basic Structure
+```typescript
+/// <reference path='fourslash.ts'/>
+
+////code goes here with /*markers*/
+
+// Test assertions go here
+```
+
+### Key Syntax Elements
+
+#### 1. Source Code Definition
+Use `////` to define source code lines:
+```typescript
+////function foo(x: number) {
+////    return x + 1;
+////}
+////let result = foo(/*marker*/42);
+```
+
+#### 2. Markers for Positioning
+Use `/**/` for anonymous markers or `/*name*/` for named markers:
+```typescript
+////let x = /*1*/someValue;
+////let y = /*cursor*/anotherValue;
+```
+
+#### 3. Multi-file Tests
+Use `// @Filename:` to define multiple files:
+```typescript
+// @Filename: /a.ts
+////export const value = 42;
+
+// @Filename: /b.ts  
+////import { value } from './a';
+////console.log(/*marker*/value);
+```
+
+#### 4. Ranges
+Use `[|text|]` to define text ranges:
+```typescript
+////function test() {
+////    [|return 42;|]
+////}
+```
+
+### Common API Patterns
+
+#### Navigation & Positioning
+```typescript
+goTo.marker("markerName");         // Navigate to marker
+goTo.marker();                     // Navigate to anonymous marker /**/
+```
+
+#### Verification (Prefer these over baselines)
+```typescript
+verify.currentLineContentIs("expected content");
+verify.completions({ includes: "itemName" });
+verify.completions({ excludes: "itemName" });
+verify.quickInfoIs("expected info");
+verify.codeFix({
+    description: "Fix description",
+    newFileContent: "expected content after fix"
+});
+```
+
+#### Completions Testing
+```typescript
+verify.completions({ 
+    marker: "1",
+    includes: { name: "foo", source: "/a", hasAction: true },
+    isNewIdentifierLocation: true,
+    preferences: { includeCompletionsForModuleExports: true }
+});
+```
+
+#### Code Fixes Testing
+```typescript
+verify.codeFix({
+    description: "Add missing property",
+    index: 0,
+    newFileContent: `class C {
+    property: string;
+    method() { this.property = "value"; }
+}`
+});
+```
+
+#### Formatting
+```typescript
+format.document();
+verify.currentLineContentIs("formatted content");
+```
+
+### Simple Example
+```typescript
+/// <reference path='fourslash.ts'/>
+
+////interface User {
+////    name: string;
+////}
+////
+////const user: User = {
+////    /*completion*/
+////};
+
+verify.completions({
+    marker: "completion",
+    includes: { name: "name", sortText: "0" }
+});
+```
+
+## Compiler Test Syntax Guide
+
+Compiler tests validate TypeScript compilation behavior, type checking, and error reporting.
+
+### Basic Structure
+- Simple `.ts` files in `tests/cases/compiler/`
+- Use comments to indicate expected behavior
+- No special test harness - just TypeScript code
+
+### Compiler Directives
+Use `// @directive: value` for compiler options:
+```typescript
+// @strict: true
+// @target: ES2015
+// @lib: ES2015,DOM
+
+let x: string = 42; // Error expected
+```
+
+### Common Directives
+```typescript
+// @strict: true/false
+// @noImplicitAny: true/false  
+// @target: ES5/ES2015/ES2020/ESNext
+// @module: commonjs/amd/es6/esnext
+// @lib: ES5,DOM/ES2015/ES2020
+// @declaration: true/false
+// @skipLibCheck: true/false
+```
+
+### Multi-file Tests
+```typescript
+// @Filename: helper.ts
+export function helper(x: number): string {
+    return x.toString();
+}
+
+// @Filename: main.ts  
+import { helper } from "./helper";
+const result = helper(42);
+```
+
+### Error Expectations
+Use comments to document expected behavior:
+```typescript
+abstract class Base {
+    abstract method(): void;
+}
+
+class Derived extends Base {
+    // Missing implementation - should error
+}
+
+new Base(); // Should error - cannot instantiate abstract class
+```
+
+### Type Testing Patterns
+```typescript
+// Test type inference
+let inferred = [1, 2, 3]; // Should infer number[]
+
+// Test type compatibility  
+type A = { x: number };
+type B = { x: number; y: string };
+let a: A = { x: 1 };
+let b: B = { x: 1, y: "hello" };
+a = b; // Should work - B is assignable to A
+b = a; // Should error - A missing property y
+```
+
+### Simple Example
+```typescript
+// Test that optional properties work correctly
+interface Config {
+    required: string;
+    optional?: number;
+}
+
+const config1: Config = { required: "test" }; // Should work
+const config2: Config = { required: "test", optional: 42 }; // Should work  
+const config3: Config = { optional: 42 }; // Should error - missing required
+```
+
+## Test Writing Best Practices
+
+### For Fourslash Tests
+1. **Prefer validation over baselines** - Use `verify.currentLineContentIs()` instead of `verify.baseline*()`
+2. **Use simple, focused examples** - Test one feature at a time
+3. **Name markers clearly** - Use descriptive marker names like `/*completion*/`
+4. **Test the simplest form first** - Start with basic cases before complex scenarios
+
+### For Compiler Tests  
+1. **Use clear file names** - Name tests after the feature being tested
+2. **Add explanatory comments** - Document expected behavior with comments
+3. **Test error cases** - Include both valid and invalid code examples
+4. **Keep tests focused** - One primary feature per test file
+
+### General Guidelines
+1. **Make tests deterministic** - Avoid random or environment-dependent behavior
+2. **Use realistic examples** - Test scenarios developers actually encounter  
+3. **Start simple** - Begin with the most basic case of a feature
+4. **Test edge cases** - Include boundary conditions and error scenarios
+
+## Running Specific Tests
+
+```bash
+# Run a specific fourslash test
+hereby runtests --tests=tests/cases/fourslash/completionForObjectProperty.ts
+
+# Run a specific compiler test  
+hereby runtests --tests=tests/cases/compiler/abstractClassUnionInstantiation.ts
+
+# Run tests matching a pattern
+hereby runtests --tests=tests/cases/fourslash/completion*.ts
+```
+
+## Important Guidelines
+
+### Test Locations
+- Only add testcases in `tests/cases/compiler` or `tests/cases/fourslash`
+- Do not write direct unit tests as they are almost never the correct test format for our repo
+
+### Performance Expectations
+- Running a set of tests may take up to 4 minutes
+- A full test run may take up to 15 minutes
+- Always run `hereby lint` and `hereby format` before you're done
+
+### Working with Issues
+- Maintainer comments in the issue should generally take priority over OP's comments
+- Maintainers might give you hints on where to start. They are not always right, but a good place to start
+
+## Recommended Workflow
+
+When fixing bugs or implementing features, follow this workflow:
+
+1. **Make a testcase that demonstrates the behavior**
+   - Run it (by itself) and review the baselines it generates to ensure it demonstrates the bug
+   - Add the test and its baselines in one commit
+
+2. **Fix the bug by changing code as appropriate**
+   - Put this fix in another commit
+
+3. **Run the test you wrote again**
+   - Ensure the baselines change in a way that demonstrates that the bug is fixed
+   - Put this baseline diff in its own commit
+
+4. **Run all other tests to ensure you didn't break anything**
+   - Some collateral baseline changes are normal
+   - Put these diffs in another commit